Sunday, September 26, 2010

Greg Curnoe Bike's Fork

For the past week or so I've been working on the Curnoe bike's fork. It's been a bit of a pain. First, the steerer/crown joint was decidedly tricky. I decided to leave the cast-in lip in place on the underside of the crown, in order to line up the bottom of the crown with the water bottle boss I'd added to the bottom of the steerer as a fender mounting point. Unfortunately, this made adding silver tricky (since the silver needed to turn a 90 degree corner before beginning its journey to the other side of the joint.) I'm not entirely sure what kind of penetration I got in the joint, but it seems to be adequate.

Then, raking the blades and brazing in the dropouts was tricky. This was my first time using the fork bending mandrel, and it was a learning experience. My first blade came out pointing off about 15 degrees off-axis — one for the scrap heap. I thought I had the the next two matched in rake, but in the fork fixture it was clear they were off by a few millimetres. This resulted in one of my dropouts being cocked a bit, but I was able to clear that up with some creative filing.

The next step was brazing on the Mafac bosses, which went smoothly enough, and then brazing in the fork blades, which actually went very well (there's the fork in the jig, about to have the blades brazed in). I did a very slow preheat, and experimented with using different flame sizes for different parts of the joint, and this was very successful. I got a lot of silver in there!

Here's a close look at the crown post-brazing. Doing the bosses first makes a certain amount of sense, since trying to brass-braze them after doing the crown/blades would melt the silver and make a mess. Speaking of messes, you can see quite a bit of burnt flux around the brake hole. Filling these holes is a gigantic pain, since you need to get the steerer and crown up to brazing temperature, and they're both massive and slow to heat. I did manage to fill the front hole, but the rear simply wouldn't go. I'll get Noah to fill it with contrasting paint instead! For future reference, I will remember to fill these holes while brazing the crown/steerer joint, when everything is already up to temperature. Also visible in the picture is my fender mounting point at the bottom of the steerer.

Here is how the fork came out. That's Greg Curnoe Bike's front wheel, a radially-laced 32-hole Mavic SSC tubular rim with a Dugast 27mm tire. The crown is a Richard Sachs Newvex that I significantly reshaped. I really like the bend our incredibly simple wooden mandrel produces. (This fork has 51mm of rake.)

A close up at the "pool-style" dropout attachment. I would call it "French-style," but it seems that French builders filed in deeper radii than I did here. I like the way this looks, and it very closely resembles the style on the original Mariposa. (By the way: the hub is a polished Deore DX [cost, about $5], and the skewer is an old Camapgnolo. I love the conical nut, and want another matching one for the rear!)

Below you can see the cool triangle-accented top of the Newvex crown (the triangles will be painted in contrasting yellow to the fork's green colour), and a mounted brake. I'm going to use a NOS Competition on the finished bike, but these well-used ones confirmed that the bosses were mounted in correct alignment—no interference between the brake arms. I left quite a bit of clearance on this fork, since I'm going to mount fenders at least some of the time. Actually, this bike will be very versatile—you could even use it for cyclocross if you were so inclined!

Of course, the bike in the painting has neither fenders nor lots of clearance. There is quite a bit of compromise, in fact, in adapting a cool-looking painting-bike to a bike I will actually want to ride (and I'll ride it a lot, since my rides are increasingly fast and shortish, and this will be a sort of 'racing' bike). The Curnoe bike is a time trial bike; mine has two chainrings, clearance for fat tires and fenders, a wider crown, Mafac bosses, etc.

But I can justify most of my changes. For instance, the tires in the Curnoe painting are bright orange. Bright orange tires do not exist in the real world—but you can quite easily have fenders painted bright orange. Having (orange) fenders not only makes the bike look more like the painting, but also makes the bike more useful. I am going to do the fastback seatstay attachment, which might seem like a weird move for a bike with fenders. The main reason to do a fastback it is that it's probably the most distinctive feature of the bike in the painting. But it's also practical for this bike; the fastback puts the seat stays close together at the brake bridge area, which means I'll be able to mount my Mafac bosses on the centre of the stays instead of on the inside, which can be a bit precarious. There should still be plenty of clearance for the 37mm Velo Orange fenders (which will indeed be orange) I'm planning on using.

There are also the tires pictured in the previous post (on Mike Barry's Greg Curnoe replica), which are I think the ones that Greg actually had on the bike. It's always a balance between form and function -- but I want to be able to use whatever tires I want... so orange fenders it is! (Which provide both adequate form, in being orange, and lots of function, in keeping road grit out of my drivetrain AND protecting what will no doubt prove an phenomenally expensive paint job!!)